From the document U.S. Pat. No. 7,095,228 a sensor system and a method for operating the sensor system are known, which allow for the analysis of two- and three-dimensional magnetic field distributions. The sensor system comprises an arrangement of sensors wherein each sensor is connected to a signal modulator. By issuing appropriate control signals, the signal modulators either forward the original sensor signal or its inverse to a signal adder, which combines all signals output by the modulators to a single system signal. The content of U.S. Pat. No. 7,095,228 is hereby incorporated by reference.
Several sensors arranged in a plane can be configured in such a way as to first measure a first component of a magnetic field in a first direction of said plane and then be configured to measure a second component of the magnetic field in a second direction of that plane, preferably orthogonal to the first direction. By performing predetermined arithmetic operations on the first and second measurements the rotational angle of the magnet with respect to the location of the sensors can be computed.
FIG. 1A shows a magnet 9, which is axially symmetric with respect to the z-axis. Its magnetic axis, i.e. the axis through its North (N) and South (S) pole is aligned with the x-axis of the coordinate-system.
FIG. 1B shows a side view of the magnet 9 along the x-axis. In addition to the magnet 9 itself, its magnetic field pattern is shown using flux lines, starting at the magnet's North (N) pole and ending at its South (S) pole.
FIG. 1C shows the amplitude B of the tangential magnetic field of the magnet 9 shown in FIG. 1A. The magnetic field amplitude is zero at the center of the magnet, i.e. the z-axis, and increases linearly to the edge of the magnet. Further away from the magnet 9 the strength of magnetic field decreases rapidly with the distance from its center and approaches zero for big distances. At a particular distance x1 from the center of the magnet, the magnetic field has a value of B0.
FIG. 2 shows the magnet 9 placed in a plane with a sensor arrangement comprising four sensors 10, 11, 12 and 13, which are placed on a circle with radius x1 around the center of the magnet 9. The angle between the symmetry axis of the magnet 9, i.e. the y-axis, and a ray through the sensor 10 is denoted with α1, which equals 45° in the presented example. In general, the rotation of the magnet 9 with respect to the sensor arrangement is denoted with the symbol α.
FIG. 3 shows a circuit comprising a magnetic sensor 10 and an associated signal modulator 30 according to the prior art. A magnetic sensor 10, for example a Hall sensor, measures the magnetic field by outputting a voltage proportional to the strength of the magnetic field on the two outputs 21 and 22. The measured voltage provided to the outputs 21 and 22 is converted into a current and amplified, if necessary, by a transconductance amplifier 70. The amplified output signal is provided to inputs 41 and 42 of the signal modulator 30. In order to keep FIG. 3 simple, no supply lines to the sensor 10, the transconductance amplifier 70 or the signal modulator 30 are shown.
The signal modulator 30 is used to forward the signal received from the inputs 41 and 42 to the modulator outputs 51 and 52, either unmodified or inverted. In order to invert the signal received from the inputs 41 and 42, the signal modulator 30 receives control signals from a control unit 60 which can either connect the output 41 with the output 51 and the output 42 with the output 52 or, inversely, connect the output 41 with the output 52 and the output 42 with the output 51.
For this purpose the signal modulator 30 comprises four electronic switches, e.g. transistors, which are controlled by three control signals en, p and m, which are provided by the control unit 60. The control signal en enables the signal modulator 30. If, together with the enable signal en, a signal p is provided to the control unit 60 the output signal from the inputs 21 and 22 is passed through to the outputs 51 and 52 unmodified. If however, a control signal m is provided in combination with a control signal en, then the inverted signal provided at the inputs 21 and 22 is forwarded to the outputs 51 and 52.
The sensor arrangement according to the prior art is constructed in such a way that always two sensors are arranged as a pair, canceling out errors resulting from a minor misalignment of the diametrically magnetized magnetic source.
However, in order to achieve optimal performance of the sensor arrangement it is important to align the magnetic source correctly in the middle of the sensor arrangement, i.e. it should be placed exactly in its center, with its magnetic axis parallel to the sensor plane and its rotary axis orthogonal to this plane and in the center of the magnet.